You can define blocks by block_name without namespace. So water_heat_sink will fall back to nuclearcraft:water_heat_sink.
Or qualify it with namespace like some_mod:some_block.
Or use block tag key. #nuclearcraft:fission_reactor_casing will fall back to blocks with this tag. Do not forget to put #.
if you need AND condition, add comma separated values "block1", "block2" means AND condition
if you need OR condition, use | separator. "block1|block2" means block1 or block2
By default you have rule condition is 'At least 1'. So if you define some block, it will go in the rule as 'at least 1'
Validation options: >2 means at least 2 (use any number)
-2 means between, it is always 2 (opposite sides)
<2 means less than 2 (use any number)
=2 means exact 2 (use any number)
^3 means 3 blocks in the corner (shared vertex or edge). possible values 2 and 3
Default placement rules have all examples